Methyl cellulose has been generally produced by the same methods as used for production of other cellulose ethers such as carboxymethyl cellulose. More specifically, methyl cellulose has been produced by the method including the steps of first immersing and mixing a raw cellulose material such as pulps in an aqueous alkali solution containing a large amount of water and an excessive amount of an alkali metal hydroxide such as sodium hydroxide to subject the cellulose to activation treatment, i.e., to so-called cellulose alkalization or mercerization for producing an alkali cellulose, and then reacting the resulting alkali cellulose with methyl chloride as an etherifying agent in a slurry condition, followed by removal of by-products such as neutralized salts by washing, drying and pulverization of the obtained product, etc.
In the cellulose alkalization step, the alkali cellulose obtained by the cellulose alkalization treatment must be subjected to complicated procedures such as washing by filtration and squeezing to remove a surplus amount of alkali or water therefrom. It is considered that a majority of hydroxyl groups contained in a molecule of the cellulose are converted into an alcoholate in the resulting alkali cellulose. In fact, the alkali cellulose contains an alkali in an amount of usually from about 1 to about 3 mol, at least 1 mol, per a glucose unit in a molecule of the cellulose. In addition, the alkali cellulose contains residual, water in an amount equal to or larger than the amount of the cellulose. Therefore, when the alkali cellulose thus obtained is allowed to react with the etherifying agent, water may also be reacted with the etherifying agent, so that not only a large amount of neutralized salts but also those compounds derived from hydrates thereof tend to be by produced.
Further, in the above reaction, since it is required to contact the solid cellulose with the etherifying agent in an efficient manner, the alkali cellulose is usually dispersed in various polar solvents to conduct the reaction in a well-dispersed slurry condition. Examples of the polar solvents added include lower secondary or tertiary alcohol solvents having a relatively low reactivity such as isopropanol, ethers and ketones. For example, in Patent Documents 1 and 2, there is described the method in which a polar solvent such as tert-butanol and methyl isobutyl ketone which is hardly water-compatible, i.e., compatible with only a small amount of water is added upon the cellulose alkalization and the reaction with the etherifying agent, and then the solvent is separated and recovered from a water phase after completion of the reaction.
However, unless any measure for considerably reducing the amounts of the alkali and residual water is taken, it is substantially difficult to reduce amounts of the by-products such as neutralized salts to a large extent.
On the other hand, there is known such an extremely classical method in which an alcohol is reacted with methanol in the presence of a strong acid such as sulfuric acid to obtain a methyl ether. Also, the methods for methyl etherification of 1,2-diols such as ethylene glycol are disclosed, for example, in Patent document 3, etc.
However, when these methods are applied to reactions of compounds having an acid-sensitive functional group such as an aldehyde, undesirable side reactions tend to readily occur. For example, polysaccharides such as celluloses readily undergo breakage of 1,4-glycosyl bond in a main chain thereof, which results in considerable reduction in molecular weight thereof.
Patent Document 4 also discloses the method in which a kaolin-based clay mineral used as a catalyst is filled in a reaction tube, and ethylene glycol and methanol are reacted at a temperature of from 200 to 300° C. while flowing the ethylene glycol and methanol in a liquid phase through the reaction tube.
In addition, Patent Document 5 discloses the method in which a methanol solution of ethylene glycol is reacted in the presence of a solid acid/base catalyst such as alkali metal-phosphorus-silicon-based composite oxides at a temperature which is from 0.9 to 1.5 times a critical temperature (Tc: 239° C. (512 K)) of methanol under a pressure which is from 0.5 to 4.5 times a critical pressure (Pc: 8.1 MPa) of methanol while flowing the solution through a tubular catalyst vessel filled with the above catalyst.
However, even if the method is applied to celluloses, since the celluloses are usually substantially undissolved in methanol, it is difficult to flow these raw materials through the catalyst while being kept in a liquid phase as described above. Further, in the reaction of a batch type, it is extremely difficult to separate the solid catalyst from the celluloses or methyl cellulose that are also in the form of a solid.
Therefore, from the industrial viewpoints, it is very useful to develop a process for catalytically producing methyl cellulose with a less amount of wastes in a convenient and efficient manner.
Patent Document 1: JP 8-245701A
Patent Document 2: JP 6-199902A
Patent Document 3: JP 61-186336A
Patent Document 4: JP 55-104221A
Patent Document 5: JP 2004-196783A